Inversionless light amplification and optical switching controlled by state-dependent alignment of molecules

We propose a method to achieve amplification without population inversion by anisotropic molecules whose orientation by an external electric field is state-dependent. It is based on decoupling of the lower-state molecules from the resonant light while the excited ones remain emitting. The suitable class of molecules is discussed, the equation for the gain factor is derived, and the magnitude of the inversionless amplification is estimated for the typical experimental conditions. Such switching of the sample from absorbing to amplifying via transparent state is shown to be possible both with the aid of dc and ac control electric fields.Comment: AMS-LaTeX v1.2, 4 pages with 4 figure

Optical properties, electron-phonon coupling, and Raman scattering of vanadium ladder compounds

The electronic structure of two V-based ladder compounds, the quarter-filled NaV$_2$O$_5$ in the symmetric phase and the iso-structural half-filled CaV$_2$O$_5$ is investigated by ab initio calculations. Based on the bandstructure we determine the dielectric tensor $\epsilon(\omega)$ of these systems in a wide energy range. The frequencies and eigenvectors of the fully symmetric A$_{g}$ phonon modes and the corresponding electron-phonon and spin-phonon coupling parameters are also calculated from first-principles. We determine the Raman scattering intensities of the A$_g$ phonon modes as a function of polarization and frequency of the exciting light. All results, i.e. shape and magnitude of the dielectric function, phonon frequencies and Raman intensities show very good agreement with available experimental data.Comment: 11 pages, 10 figure

Unusual Presentation Of Self-Limiting Chorioretinopathy Possibly Associated With COVID-19

The paper reports on an unusual presentation of self-limiting chorioretinopathy possibly associated with COVID-19 in a 47-year-old woman previously tested positive for SARS-COV-2 by PCR, who presented with decreased VA in her right eye on day 15 after testing. The only symptom she had was anosmia and аgeusia. No therapy was taken during the 14-days quarantine period. The anterior segment was unremarkable on slit lamp examination in both eyes. Fundus examination of the left eye showed no significant findings. On the other hand, the examination of the right eye revealed yellowish creamy deep chorioretinal lesion in central retina. Two weeks later the patient came back for follow-up and it was found that the lesion was decreased in size, with VA improvement. Ocular signs in COVID-19 cases are rarely reported, and most of the ophthalmological changes have been associated usually with viral conjunctivitis. Researchers report also finding such as cotton wool spots, microhemorrhages and some OCT-changes. This case illustrates the possible association between COVID-19 and retinopathy despite the almost asymptomatic presentation

Inversionless gain in a three-level system driven by a strong field and collisions

Inversionless gain in a three-level system driven by a strong external field and by collisions with a buffer gas is investigated. The mechanism of populating of the upper laser level contributed by the collision transfer as well as by relaxation caused by a buffer gas is discussed in detail. Explicit formulae for analysis of optimal conditions are derived. The mechanism developed here for the incoherent pump could be generalized to other systems.Comment: RevTeX, 9 pages, 4 eps figure

Folded modes in the infrared spectra of the spin-Peierls phase of CuGeO_3

Polarized far-infrared transmittance spectra of CuGeO_3 single crystals were measured at different temperatures (6K < T < 300K). Two spectral lines, at 284.2 cm-1 in E||c polarization and at 311.7 cm-1 in E||b polarization, appear at the temperature of the spin-Peierls transition and grow in intensity with decreasing temperature. Both of them are, most probably, folded modes of the dimerized lattice. We discuss a possible role of the spin-phonon interaction in the formation of the 311.7 cm-1 feature.Comment: 4 pages, 5 figures, 1 table; Submitted to Phys.Rev.B Second revision. Figures and text were slightly change

Positive Semidefiniteness and Positive Definiteness of a Linear Parametric Interval Matrix

We consider a symmetric matrix, the entries of which depend linearly on some parameters. The domains of the parameters are compact real intervals. We investigate the problem of checking whether for each (or some) setting of the parameters, the matrix is positive definite (or positive semidefinite). We state a characterization in the form of equivalent conditions, and also propose some computationally cheap sufficient\,/\,necessary conditions. Our results extend the classical results on positive (semi-)definiteness of interval matrices. They may be useful for checking convexity or non-convexity in global optimization methods based on branch and bound framework and using interval techniques

Charge ordering in quarter-filled ladder systems coupled to the lattice

We investigate charge ordering in the presence of electron-phonon coupling for quarter-filled ladder systems by using Exact Diagonalization. As an example we consider NaV2O5 using model parameters obtained from first-principles band-structure calculations. The relevant Holstein coupling to the lattice considerably reduces the critical value of the nearest-neighbor Coulomb repulsion at which formation of the zig-zag charge-ordered state occurs, which is then accompanied by a static lattice distortion. Energy and length of a kink-like excitation on the background of the distorted lattice are calculated. Spin and charge spectra on ladders with and without static distortion are obtained, and the charge gap and the effective spin-spin exchange parameter J are extracted. J agrees well with experimental results. Analysis of the dynamical Holstein model, restricted to a small number of phonons, shows that low frequency lattice vibrations increase the charge order, accompanied by dynamically produced zig-zag lattice distortions.Comment: 11 pages, 17 figures, revised version as to appear in Phys. Rev.